1. Field of the Invention
This invention generally relates to a linear motion guide unit and its manufacturing method, and, in particular, to a linear motion guide unit having a vibration resistant mechanism and its method of manufacturing. More specifically, the present invention relates to an anti-vibration linear motion guide unit which is particularly suited for use in guiding a linear motion in machine tools, industrial robots or the like.
2. Description of the Prior Art
In processing or machining a work piece by machine tools, either one of the work piece or machine tools is held stationary and the other is caused to move, and a linear motion guide unit may be used in guiding a linear motion in such a case. The linear motion guide unit typically includes a rail extending straight over a desired length and a slider slidably mounted on the rail. The linear motion guide unit may be either of the rolling contact type or of the sliding contact type. In the former type, a plurality of rolling members, such as rollers or balls, are provided between the rail and the slider to thereby provide a rolling contact between the rail and the slider. On the other hand, in the latter case, a sliding member is provided between the rail and the slider to thereby provide a sliding contact therebetween. The sliding contact type linear motion guide unit is typically used for providing a damping or vibration resistant characteristic.
Such a linear motion guide unit has various applications including machine tools. One example of conventional machine tool applications is illustrated in FIGS. 11 through 13. As shown in FIGS. 11 and 12, in this particular machine tool application, a rail 53 is fixedly attached to a base 51 by means of bolts 52 and on the rail 53 are slidably mounted a pair of end sliders 55 and a center slider 56 located between the pair of end sliders 55. As will be made clear later, in this particular example, the pair of end sliders 55 are of the rolling contact type sliders and the center slider 56 is of the sliding contact type slider having an anti-vibration mechanism. All of these end and center sliders 55 and 56 are fixedly attached to a table 58 by means of bolts 59. As a result, the table 58 may move linearly back and forth along the rail 53 relative to the base 51 as indicated by a double arrow F.
As best shown in FIG. 12, the rail 53 has a pair of upper and lower guide surfaces 53a on each side thereof, which extend in parallel with the longitudinal axis of the rail 53. The rail 53 also has a bottom mounting surface 53b which sits on the top surface of the bed 51 and a top flat surface 53c. The rail 53 is provided with a plurality of mounting holes 53d at a predetermined pitch and each of the mounting holes 53d extends through the rail 53 from its flat top surface 53c to its bottom surface and has a stepped portion serving as a stopper for the head of the bolt 52. The stepped portion is so located that the head of the mounting bolt 52 is completely received in the mounting hole 53d so as not to interfere with any of the sliders 55 and 56.
As shown in FIG. 12, the end slider 55 is slidably mounted on the rail 53 in a straddling manner and provided with four endless circulating paths 55a, each including a load path section, a return path section and a pair of curved connecting path sections, each connecting the corresponding ends of the load and return path sections, as well known in the art. The load path section is defined by a pair of the guide surface 53a of the rail 53 and its oppositely located outer guide surface formed in the slider 55. A plurality of rolling members or rollers 60 in this example are provided in each of the four endless circulating paths 55a. Those rollers 60 which are located in the load path sections of the endless circulating paths 55a provide a rolling contact between the rail 53 and the slider 55.
FIG. 13, on the other hand, illustrates how the center slider 56 is slidably mounted on the rail 53. As shown, the center slider 56 is also slidably mounted on the rail 53 in a straddling manner with a small buffer gap e.sub.1 provided between the rail 53 and the center slider 56. The buffer gap e.sub.1 is filled with an oil supplied through a supply passage 56a so that a film of oil or grease 63, which serves to damp or absorb vibration occurring between the rail 53 and the slider 56, is defined in this buffer gap e.sub.1. Thus, the center slider 56 is provided with a vibration resistant mechanism which includes the film of oil 63. As a result, when the table 58 moves back and forth relative to the base 51 in the direction indicated by the double arrow F, any vibration which may occur between the base 51 and the table 58 can be advantageously absorbed or damped so that the table 58 may move relative to the base 51 virtually without vibration.
In the structure shown in FIG. 11, the table 58 is fixedly mounted on the single center slider 56 and two end sliders 55. However, in another structure, the table 58 may be fixedly attached to another set of center and end blocks which are slidably mounted on a rail extending in parallel with the rail 53 and fixedly mounted on the base 51 commonly. In either structure, a work piece to be processed or machined (not shown) may be fixedly mounted on the table 58 temporarily and may be processed or machined while moving back and forth in the direction indicated by the double arrow F.
As described above, since the center slider 56 is provided with an anti-vibration mechanism, the table 58 may move relative to the base 51 without vibration. However, in the prior art structure shown in FIG. 13, when the oil 63 changes its characteristics after use for some time, for example, due to mechanical vibration, heat and/or aging, the oil 63 tends to leak away from the gap. Therefore, it is required to supply additional oil 63 from time to time or in the worst case constantly. In addition, the oil 63 is also supplied into the mounting hole 53d and some of the oil 63 may also leak through a clearance between the bolt 52 and the mounting hole 53d. Moreover, in the prior art structure shown in FIG. 13, difficulty was encountered in keeping a the buffer gap e.sub.1 at an optimal value and also in distributing the oil 63 uniformly across the top surface 53c of the rail 53 since the oil was only supplied from the supply passage 56a.